Multiple layers of heterogeneity and subset diversity in human MAIT cell responses to distinct microorganisms and to innate cytokines. Proc Natl Acad Sci U S A. 2017;114:E5434-E5443. [PMID: 28630305 DOI: 10.1073/pnas.1705759114]

Mucosal-Associated Invariant T Cells in Chronic Inflammatory Liver Disease

The broadening field of microbiome research has led to a substantial reappraisal of the gut-liver axis and its role in chronic liver disease. The liver is a central immunologic organ that is continuously exposed to food and microbial-derived antigens from the gastrointestinal tract. Mucosal-associated invariant T (MAIT) cells are enriched in the human liver and can be activated by inflammatory cytokines and microbial antigens. In chronic inflammatory liver disease, MAIT cells are depleted suggesting an impaired MAIT cell-dependent protection against bacterial infections.

MAIT cells are activated in acute Dengue virus infection and after in vitro Zika virus infection

Dengue virus (DENV) and Zika virus (ZIKV) are members of the Flaviviridae and are predominantly transmitted via mosquito bites. Both viruses are responsible for a growing number of infections in tropical and subtropical regions. DENV infection can cause lethargy with severe morbidity and dengue shock syndrome leading to death in some cases. ZIKV is now linked with Guillain-Barré syndrome and fetal malformations including microcephaly and developmental disorders (congenital Zika syndrome). The protective and pathogenic roles played by the immune response in these infections is unknown. Mucosal-associated invariant T (MAIT) cells are a population of innate T cells with potent anti-bacterial activity. MAIT cells have also been postulated to play a role in the immune response to viral infections. In this study, we evaluated MAIT cell frequency, phenotype, and function in samples from subjects with acute and convalescent DENV infection. We found that in acute DENV infection, MAIT cells had elevated co-expression of the activation markers CD38 and HLA-DR and had a poor IFNγ response following bacterial stimulation. Furthermore, we found that MAIT cells can produce IFNγ in response to in vitro infection with ZIKV. This MAIT cell response was independent of MR1, but dependent on IL-12 and IL-18. Our results suggest that MAIT cells may play an important role in the immune response to Flavivirus infections.

Ontogeny of human mucosal-associated invariant T cells and related T cell subsets

There are very few human MAIT cells in cord blood. Ben Youssef et al. show that they slowly expand during childhood and point to a critical role of the TCRαβ repertoire in determining their unique ability to recognize MR1-restricted microbial antigens.

Mucosal-associated invariant T (MAIT) cells are semi-invariant Vα7.2⁺ CD161^highCD4⁻ T cells that recognize microbial riboflavin precursor derivatives such as 5-OP-RU presented by MR1. Human MAIT cells are abundant in adult blood, but there are very few in cord blood. We longitudinally studied Vα7.2⁺ CD161^high T cell and related subset levels in infancy and after cord blood transplantation. We show that Vα7.2⁺ and Vα7.2⁻ CD161^high T cells are generated early during gestation and likely share a common prenatal developmental program. Among cord blood Vα7.2⁺ CD161^high T cells, the minority recognizing MR1:5-OP-RU display a TRAV/TRBV repertoire very similar to adult MAIT cells. Within a few weeks of life, only the MR1:5-OP-RU reactive Vα7.2⁺ CD161^high T cells acquire a memory phenotype. Only these cells expand to form the adult MAIT pool, diluting out other Vα7.2⁺ CD161^high and Vα7.2⁻ CD161^high populations, in a process requiring at least 6 years to reach adult levels. Thus, the high clonal size of adult MAIT cells is antigen-driven and likely due to the fine specificity of the TCRαβ chains recognizing MR1-restricted microbial antigens.

MAIT cells: an historical and evolutionary perspective

In humans, MAIT cells represent the most abundant T-cell subset reacting against bacteria. MAIT cells belong to the evolutionarily conserved family of "preset" T cells that includes also NKT cells. Both subsets are selected by double positive thymocytes leading to common features such as PLZF expression. Preset T cells correspond to subsets prepositioned in specific tissue locations with preprogrammed versatile effector functions such as antimicrobial functions and possibly also metabolic control and tissue repair activity. Herein, we recall how several groups studying human samples discovered MAIT cells as T cells expressing either a restricted T-cell receptors (TCR) repertoire or homogeneous and singular phenotypic and functional characteristics. We then highlight the main evolutionary features of this subset and its restricting element, MR1 (MHC-related protein (1) with a striking coevolution of TRAV1 and MR1. We introduce another evolutionarily conserved invariant TCRalpha chain coevolving with another MHC class Ib molecule, called MHX, sharing phylogenetic features with MR1. We finally discuss the relationship between MAIT cells and other subsets reacting to microbial antigens or to compounds presented by MR1 in light of confounding experimental issues.

Mucosal-Associated Invariant T Cells Display a Poor Reconstitution and Altered Phenotype after Allogeneic Hematopoietic Stem Cell Transplantation

Mucosal-associated invariant T (MAIT) cells are innate-like T cells which are important in the defense against certain bacteria and yeast. The reconstitution of MAIT cells after allogeneic hematopoietic stem cell transplantation (HSCT) is not known. We investigated MAIT cell phenotype and function in 17 patients devoid of relapse and severe graft-versus-host disease (GvHD) in paired samples collected 1-2, 3-6, 12, and 24 months after transplantation. Data were compared to 17 healthy controls (HC), as well as 22 patients with acute GvHD grade 2-3. The frequency of MAIT cells within CD3⁺ cells was approximately 10-fold lower than in HC and did not increase over the 2 years following HSCT. MAIT cells in HSCT patients displayed an elevated expression of CD69 and intracellular granzyme B and were predominantly composed of CD4/CD8 double-negative cells. The expression of PD-1 on MAIT cells was low and did not change during the observational time, whereas the CD3⁺CD161^dim/negTCRVα7.2^dim/neg cells (non-MAIT T cells) displayed a high expression early after HSCT that decreased to normal levels at 24 months. MAIT cells collected 2-6 months post-HSCT showed an impaired IFN-γ and perforin response after bacterial stimulation, but the response was restored at 24 months. Patients with acute GvHD had similar proportions of MAIT cells as patients with grade 0-1, but consisted mainly of CD8⁺ cells. Finally, MAIT cells were more sensitive to cyclosporine A and sirolimus than non-MAIT T cells. To conclude, MAIT cell reconstitution following HSCT is deficient compared to non-MAIT T cells and GvHD grade ≥2 is not correlated with MAIT cell frequency. MAIT cell functionality was impaired early after HSCT, but restored at 24 months post-HSCT. MAIT cells have an increased sensibility to common immunosuppressive drugs, which maybe could explain their hampered reconstitution after HSCT.

Interactions between the microbiota and innate and innate-like lymphocytes

The microbiota, which consists of commensal bacteria, fungi, and viruses, limits the colonization of pathogens at barrier tissues and promotes immune homeostasis. The latter is accomplished through the induction and regulation of both innate and adaptive immune responses. Innate lymphocytes, which include the type-1 innate lymphoid cell (ILC1), NK cell, type-2 innate lymphoid cell (ILC2), type-3 innate lymphoid cell (ILC3), and lymphoid tissue inducer (LTi) cell populations, and innate-like lymphocytes, such as NKT cells, mucosal-associated invariant T (MAIT) cells, and γδ T cells, are uniquely capable of responding to the microbiota due to their tissue localization and rapid primary responses. In turn, through their effector functions, these lymphocyte populations modulate the composition of the microbiota and maintain the segregation of commensals. This review will focus on how innate and innate-like lymphocytes mediate the crosstalk with the microbiome.

Mucosal-Associated Invariant T Cell Interactions with Commensal and Pathogenic Bacteria: Potential Role in Antimicrobial Immunity in the Child

Mucosal-associated invariant T (MAIT) cells are unconventional CD3⁺CD161^high T lymphocytes that recognize vitamin B2 (riboflavin) biosynthesis precursor derivatives presented by the MHC-I related protein, MR1. In humans, their T cell receptor is composed of a Vα7.2-Jα33/20/12 chain, combined with a restricted set of Vβ chains. MAIT cells are very abundant in the liver (up to 40% of resident T cells) and in mucosal tissues, such as the lung and gut. In adult peripheral blood, they represent up to 10% of circulating T cells, whereas they are very few in cord blood. This large number of MAIT cells in the adult likely results from their gradual expansion with age following repeated encounters with riboflavin-producing microbes. Upon recognition of MR1 ligands, MAIT cells have the capacity to rapidly eliminate bacterially infected cells through the production of inflammatory cytokines (IFNγ, TNFα, and IL-17) and cytotoxic effector molecules (perforin and granzyme B). Thus, MAIT cells may play a crucial role in antimicrobial defense, in particular at mucosal sites. In addition, MAIT cells have been implicated in diseases of non-microbial etiology, including autoimmunity and other inflammatory diseases. Although their participation in various clinical settings has received increased attention in adults, data in children are scarce. Due to their innate-like characteristics, MAIT cells might be particularly important to control microbial infections in the young age, when long-term protective adaptive immunity is not fully developed. Herein, we review the data showing how MAIT cells may control microbial infections and how they discriminate pathogens from commensals, with a focus on models relevant for childhood infections.

Mucosal-Associated Invariant T Cells: New Insights into Antigen Recognition and Activation

Mucosal-associated invariant T (MAIT) cells, a novel subpopulation of innate-like T cells that express an invariant T cell receptor (TCR)α chain and a diverse TCRβ chain, can recognize a distinct set of small molecules, vitamin B metabolites, derived from some bacteria, fungi but not viruses, in the context of an evolutionarily conserved major histocompatibility complex-related molecule 1 (MR1). This implies that MAIT cells may play unique and important roles in host immunity. Although viral antigens are not recognized by this limited TCR repertoire, MAIT cells are known to be activated in a TCR-independent mechanism during some viral infections, such as hepatitis C virus and influenza virus. In this article, we will review recent works in MAIT cell antigen recognition, activation and the role MAIT cells may play in the process of bacterial and viral infections and pathogenesis of non-infectious diseases.

Role of MAIT cells in the immunopathogenesis of inflammatory diseases: New players in old game

Current advances in immunology have led to the identification of a population of novel innate immune T cells, called mucosa-associated invariant T (MAIT) cells. The cells in humans express an invariant TCRα chain (Vα7.2-Jα33) paired with a limited subset of TCRβ chains (Vβ2, 13 and 22), are restricted by the MHC class I (MH1)-related (MR)-1, and recognize molecules that are produced in the bacterial riboflavin (vitamin B2) biosynthetic pathway. They are present in the circulation, liver and at various mucosal sites (i.e. intestine, lungs and female reproductive tract, etc.). They kill host cells infected with bacteria and yeast, and secrete soluble mediators such as TNF-α, IFN-γ, IL-17, etc. The cells regulate immune responses and inflammation associated with a wide spectrum of acute and chronic diseases in humans. Since their discovery in 1993, significant advances have been made in understanding biology of MAIT cells and the potential role of these cells in the pathogenesis of autoimmune, inflammatory and infectious diseases as well as cancer in humans. The purpose of this review is to provide a current state of our knowledge about MAIT cell biology and delineate their role in autoimmune and inflammatory diseases (sterile or caused by infectious agents) and cancer in humans. A better understanding of the role of MAIT cells in human diseases may lead to novel ways of immunotherapies.

The MAIT conundrum - how human MAIT cells distinguish bacterial colonization from infection in mucosal barrier tissues

We review the recent human mucosal-associated invariant T (MAIT) cell literature to examine the signals that control MAIT cell activation. We discuss these signals in context of MAIT cell function in mucosal barrier tissues and address how MAIT cells avoid responding to commensal bacteria, while maintaining responsiveness to infections.

Activation of Human Mucosal-Associated Invariant T Cells Induces CD40L-Dependent Maturation of Monocyte-Derived and Primary Dendritic Cells

Mucosal-associated invariant T (MAIT) cells are innate T cells that recognize intermediates of the vitamin B2 biosynthetic pathway presented by the monomorphic MR1 molecule. It remains unclear whether, in addition to their cytolytic activity that is important in antimicrobial defense, MAIT cells have immune-modulatory functions that could enhance dendritic cell (DC) maturation. In this study, we investigated the molecular mechanisms dictating the interactions between human MAIT cells and DCs and demonstrate that human MAIT cells mature monocyte-derived and primary DCs in an MR1- and CD40L-dependent manner. Furthermore, we show that MAIT cell–derived signals synergize with microbial stimuli to induce secretion of bioactive IL-12 by DCs. Activation of human MAIT cells in whole blood leads to MR1- and cytokine-dependent NK cell transactivation. Our results underscore an important property of MAIT cells, which can be of translational relevance to rapidly orchestrate adaptive immunity through DC maturation.

Bacterial deception of MAIT cells in a cloud of superantigen and cytokines

The bacterium Staphylococcus aureus is an important cause of the life-threatening condition toxic shock syndrome in humans. Bacterial toxins known as superantigens (SAgs) generate this illness by acting as broad activators of a substantial fraction of all T lymphocytes, bypassing the normally highly stringent T-cell receptor antigen specificity to cause a systemic inflammatory cytokine storm in the host. In a new study, Shaler et al. found that immune cells called mucosa-associated invariant T (MAIT) cells make an unexpectedly large contribution to the SAg response in a largely T-cell receptor–independent, cytokine-driven manner. Subsequent to such activation, the MAIT cells remain unresponsive to stimulation with bacterial antigen. Thus, S. aureus hijacks MAIT cells in the cytokine storm and leaves them functionally impaired. This work provides new insight into the role of MAIT cells in antibacterial immunity and opens new avenues of investigation to understand and possibly treat bacterial toxic shock and sepsis.

Gut-CNS-Axis as Possibility to Modulate Inflammatory Disease Activity-Implications for Multiple Sclerosis

In the last decade the role of environmental factors as modulators of disease activity and progression has received increasing attention. In contrast to classical environmental modulators such as exposure to sun-light or fine dust pollution, nutrition is an ideal tool for a personalized human intervention. Various studies demonstrate a key role of dietary factors in autoimmune diseases including Inflammatory Bowel Disease (IBD), rheumatoid arthritis or inflammatory central nervous system (CNS) diseases such as Multiple Sclerosis (MS). In this review we discuss the connection between diet and inflammatory processes via the gut-CNS-axis. This axis describes a bi-directional communication system and comprises neuronal signaling, neuroendocrine pathways and modulation of immune responses. Therefore, the gut-CNS-axis represents an emerging target to modify CNS inflammatory activity ultimately opening new avenues for complementary and adjunctive treatment of autoimmune diseases such as MS.